1. Field of the Invention
The present invention relates generally to solar cells, and more particularly but not exclusively to solar cell fabrication processes and structures.
2. Description of the Background Art
Solar cells are well known devices for converting solar radiation to electrical energy. They may be fabricated on a semiconductor substrate using semiconductor processing technology. A solar cell includes P-type and N-type diffusion regions. Solar radiation impinging on the solar cell creates electrons and holes that migrate to the diffusion regions, thereby creating voltage differentials between the diffusion regions. In a backside contact solar cell, both the diffusion regions and the metal contact fingers coupled to them are on the backside of the solar cell. The contact fingers allow an external electrical circuit to be coupled to and be powered by the solar cell.
Backside contact solar cells, in general, are known in the art. Examples of backside contact solar cells are disclosed in U.S. Pat. Nos. 5,053,083 and 4,927,770, which are both incorporated herein by reference in their entirety. FIG. 1 schematically shows another example of a conventional backside contact solar cell.
In the example of FIG. 1, a conventional backside contact solar cell 100 includes an N-type silicon substrate 102. The front side of the solar cell 100 is generally labeled as 120 and the backside, which is opposite the front side, is generally labeled as 121. The front side of the solar cell faces the sun during normal operation to collect solar radiation. The front side is randomly textured to reduce reflection and thereby increase the amount of solar radiation collected in the substrate 102. A multilayer anti-reflection structure 110 comprising a thermally grown silicon dioxide (SiO2) layer 122 and a silicon nitride layer 103 is formed on the textured silicon surface.
The backside of the solar cell 100 includes P-type diffusion regions 105 and N-type diffusion regions 106. The diffusion regions 105 and 106 may be formed by diffusion of appropriate dopants from the backside. Metal fingers 109 electrically connect to the P-type diffusion regions 105, while metal fingers 110 electrically connect to the N-type diffusion regions 106. The metal fingers 109 and 110 allow electrons generated in the solar cell 100 to be utilized by external electrical circuits. Layers 107 provide isolation to prevent electrical shorts.
The performance of a backside contact solar cell improves as the interface state density between SiO2 and Si is reduced. The interface between the silicon dioxide layer 122 and the surface of the substrate 102 is thus designed to reduce their interface state density. Silicon nitride layer 103 may also further reduce the effect of the SiO2/Si interface states on the performance of the solar cell 100. The process of reducing the SiO2/Si interface state density and their effect on solar cell performance is also referred to as “passivation.”
Embodiments of the present invention help prevent degradation of front side passivation of a backside contact solar cell.